The present invention relates to a method for the preparation of a high density sintered body of silicon carbide or, more particularly, to a method for the preparation of a high density sintered body of silicon carbide by use of a specific sintering aid as well as a sintered body of silicon carbide prepared by the method.
As is well known, sintered bodies of silicon carbide are very promising as a refractory material having excellent oxidation resistance, corrosion resistance, heat-shock resistance and mechanical strengths, in particular, at high temperatures so that they are used as a high-temperature structural body, anti-abrasion material, friction member and the like.
Sintered bodies of silicon carbide are usually prepared by admixing powdery silicon carbide with a sintering aid and a binder followed by shaping and sintering at an elevated temperature. Selection of the sintering aid is very important in order to obtain sintered bodies of silicon carbide having satisfactory properties or, in particular, high density. For example, boron carbide is the most widely used sintering aid but the extremely high hardness of this material causes considerable difficulties in pulverizing the material into fine particles to present a serious problem in the industrial production of silicon carbide sintered bodies. With an object to overcome this problem, a method has been proposed in which boron oxide, capable of being converted into boron carbide in the sintering process, is used as the sintering aid in place of boron carbide by utilizing the pulverizability inherent to boron oxide. Unfortunately, however, the boiling point of boron oxide is relatively low at 1860.degree. C. so that a part of the boron oxide admixed with silicon carbide is lost by dissipation before the sintering temperature is reached and the desired effect of sintering aid can not always be obtained.
On the other hand, it is known that aluminum and aluminum compounds may serve as a sintering aid for silicon carbide. Metallic aluminum in a finely pulverized form, however, is disadvantageous because it is highly pyrophoric and must be handled with utmost carefulness in addition to the low dispersibility not to ensure uniform dispersion in the silicon carbide powder due to agglomeration readily taking place in aluminum powders. As to the aluminum compounds as a sintering aid, few of the compounds are sufficiently active as themselves. Moreover, sulfate and nitrate of aluminum capable of giving active aluminum oxide have a problem that toxic gases are produced in the course of sintering in the form of sulfur oxides or nitrogen oxides while aluminum-containing organic compounds such as aluminum isopropoxide, aluminum citrate, aluminum monostearate and the like must be used in a relatively large amount in order to give a sufficient amount of alumina due to the low content of aluminum therein so that the shaped body under sintering is subject to a great shrinkage in volume due to the decomposition of the aluminum compound in the course of sintering and the sintered bodies obtained therewith cannot be dense enough.
Thus, none of the conventionally used sintering aids for silicon carbide is satisfactory when high-density sintered bodies of silicon carbide are desired.